Diamond is an allotrope of carbon which is metastable at ordinary pressures, having a large activation energy barrier which prevents conversion to graphite, which is the more stable allotrope at ordinary temperatures and pressures. In addition to its value as a precious gem, the uses of diamond include many industrial applications such as use in polishing, grinding, sawing and drilling, as windows in high pressure cells, in diamond microtone knives for biological and medicinal uses, as a radiation detector, for temperature measurement, as heat sinks, as wire drawing dyes, styli for photographs and as hardness indenters. Thus numerous approaches have been utilized to attempt to synthesize diamond.
One approach is to utilize high pressure methods since at high pressures diamond, rather than graphite, is the thermodynamically stable form of carbon. However, heretofore, high pressure methods have met only with limited commercial success since only small diamond crystals have been made, which are suitable mainly for use as abrasives and in forming sintered preforms for use as wire drawing dyes or tool bits. Moreover, the product of high pressure diamond synthesis is often contaminated with impurities of more or less uncontrollable concentration and distribution, rendering such diamond unsuitable for a number of important technical applications.
There have been attempts to grow diamond under low pressure conditions which, at first impression, may seem to be against thermodynamic principles. However, upon crystallization at low pressures using free carbon atoms, the carbon atoms, during their fall from a state of higher free energy, may be made to pause (i.e. crystallize) at the level of diamond, instead of going to graphite. Therefore free carbon atoms with a higher free energy than that in diamond may be made to crystallize as diamond under suitable conditions, thus the metastability of diamond alone is not a deterrent factor of obtaining diamond at atmospheric or reduced pressure. Moreover, metastable phases, such as diamond, may be made to grow in the stability field of another phase, when nucleation and growth is facilitated by providing seeds of the required phase or a substrate which allows epitaxial overgrowth. Thus there are numerous techniques utilizing low pressure epitaxial crystallization of diamond. However, these gas phase synthesis techniques suffer from the problems of extremely low growth rates and/or the inevitable problem of interruption of growth due to formation of graphite. Once the graphite is formed, being the favored thermodynamic product at low pressure, it overtakes and inhibits further diamond growth. In order to maximize the time available for diamond growth before the appearance of graphite, the vapor pressure of the carbon bearing gas has been usually kept quite low, thus leading to very slow diamond deposition rates, typically about 0.1 micron/hour.
Another approach for diamond synthesis has been use of low pressure liquid phase synthesis. Liquid phase synthesis however has not as yet permitted synthesis of large, high quality diamond crystals and the products have been limited to microscopic deposits of uncertain identity. The commercial utility of liquid phase synthesis has been limited by the same problems which affect low pressure vapor phase synthesis, i.e., failure to exclude poison species (such as water), lack of species which stabilize the carbon sp.sup.3 bonds (such as atomic hydrogen), and failure to develop process conditions which form high quality crystals at acceptable rates of growth.
There are many processes known in the art in which diamond is synthesized under high pressure at which diamond is the thermodynamically stable form of carbon. Although there are many variations of this technique, a typical technique involves use of a suitable carbon solvent such as a transition metal alloy, and a carbon source which are compressed and heated in an apparatus capable of providing pressures of at least 60 kilobars at temperatures above 1500.degree. C. The carbon is dissolved, transported, and deposited as diamond crystals but the carbon transport rate is governed primarily by diffusion, and therefore is very low. Thus the growth rates are slow and long deposition times are required to grow large diamonds.
Furthermore because of the high pressures and temperatures required, the apparatus is necessarily bulky and expensive. Furthermore, because of the relatively small active of volume, high pressure deposition precludes effective use of mechanical techniques (such as stirring) which might improve growth rates and product quality.
All of the above methods suffer from the problem of growth of graphite, which eventually causes diamond growth to cease.
It is therefore an object of the present invention to provide a method for low pressure vapor phase metastable diamond synthesis whereby growth of graphite and other non-diamond carbon species is suppressed by evaporation or selective photolysis.
These and other objects of the present invention will be apparent to those skilled in the art from the following description of the preferred embodiments and from the appended claims.